soil management (s.chem. 4413)
TRANSCRIPT
1
PRACTICAL MANUAL
FOR
SOIL MANAGEMENT (S.CHEM. 4413)
Dr. B.L. Yadav
(Professor & Head)
(Associate Professor)
2013
DEPARTMENT OF SOIL SCIENCE AND AGRICULTURAL CHEMISTRY S.K.N. COLLEGE OF AGRICULTURE
(S.K.N. AGRICULTURE UNIVERS ITY , JOBNER)
JOBNER-303 329 (RAJ.)
2
PRACTICAL MANUAL
FOR
SOIL MANAGEMENT
(S.CHEM. 4413)
Dr. B.L. Yadav
(Professor & Head) Department of Soil Science and Agricultural Chemistry
S.K.N. College of Agriculture (S.K. N. Agriculture University, Jobner)
Jobner-303 329
Name of Student : ……………………………………………….
Class & Batch : ……………………………………………….
Session : ……………………………………………….
Name of College : ……………………………………………….
3
S. No. Exercise Page
No.
Date Signature of
Teacher
1. Collection and preparation of soil samples
for analysis
1
2. Preparation of saturation paste and extraction of soluble salts and
determination of ECe and pHs
4
3. Determination of Ca+2 + Mg+2 in saturation extract of soil by Versenate (EDTA) method
5
4. Determination of Ca2+ in saturation extract by Versenate (EDTA) method
8
5. Determination of Na+ in saturation extract of soil by flame photometer
11
6 Determination of K+ in saturation extract
of soil by flame photometer
14
7 Determination of CO32--HCO3
- in saturation extract of soil
16
8 Determination of Cl- in saturation extract
of soil
18
9 Determination of gypsum requirement of sodic soils (Schoonover, 1952)
20
10 Determination of lime requirement of acid
soils
23
11 Determination of saturated hydraulic conductivity of an undisturbed soil
25
12 Determination of bulk density of an
undisturbed soil by core sampler method
28
13 Determination of infiltration rate of soil by double ring infiltrometer
30
14 Determination of maximum water holding
capacity of the soil
34
15 Determination of field capacity (FC) of soil in field condition
36
16 Determination of available water storage capacity of soil by pressure plate
(membrane) apparatus (Richard, L.A. Weaver, L.R. 1943 and Richards, 1947).
38
17 Measurement of oxygen diffusion rate by
platinum microelectrode method (Lemon and Erickson, 1952)
42
18 Estimation of water stable soil aggregates 47
INDEX
4
Exercise [1] : Collection and Preparation of Soil Samples for Analysis
Tools and Materials
1. Different soil sampling equipments like soil tube auger, screw type auger,
post-hole auger, Khassi (spade) and Khurpi are used for taking samples.
2. For sampling of soft and moist soil the tube auger, spade or khurpi can be
used satisfactorily. A screw type auger may prove more convenient on
hard/dry soil while the post hole auger is useful for sampling wet area like
rice fields. Tools for collecting the samples should be free from rust or
any foreign material which may contaminate the samples.
3. A bucket for collecting and mixing the composite sample.
4. Clean, well-labeled bags of size 13 cm x 25 cm for storing the soil sample.
Sampling for soil reclamation
On saline and alkali soils, samples can be taken by either using a soil
auger or digging a 90 cm deep pit. In case a pit is dug, the soil samples should be
collected as follows:
1. Make one side of the pit vertical and put mark on it at 15, 30, 60 and 90
cm depth from the surface.
2. Hold a suitable container at 15 cm mark, and scrap off a uniform slice of
soil from the surface down to this mark and collect about 500 g of the soil
sample. Transfer the soil sample to a cloth bag and mark it as 0-15 cm.
3. Similarly, collect 500 g soil sample from each layer, i.e. 15-30, 30-60 and
60-90 cm and put them separately in three cloth bags after drying in
shade.
5
4. Take a separate sample of the surface crust also, if any
5. Prepare two label for each sample showing the depth from which sample
has been taken, name of farmer, name of village, exact location of the
field and condition/ growth of crop, if any.
6. Put up one label inside the bag and the other on the bag, Label should be
written with a copying pencil.
7. Information sheet may also be prepared if necessary.
Precautions
1. Sampling should be done from a uniform piece of land.
2. Each hectare land should be represented by atleast 2-3 pits.
3. If there is hard pan in the pit, it should be sampled separately and also
note its depth and thickness.
Precautions in collection of soil samples
1. Sampling bags and bottle must be free from all sorts of contamination.
2. While sampling for micronutrients auger metal should not constitute
the element desired.
3. No. of samples to be collected should be decided as per intensity of the
problem and representation of the area concerned.
4. Samples must be properly tagged with complete description of the
sampling site.
5. Elapsed time between collection and analysis of the sample should be
shorter in order to get more reliable analytical data.
6
Recording and maintenance of sample details
Each soil sample should be properly entered in the lab. register along with
the relevant information.
The information sheet usually used in laboratories is shown below:
Date------------------------------------
Lab. No.------------------------------- Farmer’s name------------------------------
Village -------------------------------- Distt. and State----------------------------
Topography--------------------------- Natural vegetation-------------------------
Present land use---------------------- Distance from river and Nala-------------
Profile characteristics ---------------- Source of water-----------------------------
Irrigation details ---------------------- Fertilizer use--------------------------------
Drainage ------------------------------- Farmers opinion----------------------------
7
Exercise [2]:Preparation of saturation paste and extraction of soluble salts
and determination of ECe and pHs
Principle
The water soluble salts in soils are generally determined by two type of
soil water extracts, (i) saturation extract, (ii) 1:2 soil water extract. If EC of soil
saturation extract is greater than 2 dSm-1
(or more than 1.0 dSm-1
in 1:2 soil
water extract), the water extract should be retained for determination of soluble
ions. The saturation extract considered more reliable because, it is directly
related to the field moisture range. However, determination in saturation extract
is time consuming, 1:2 soil water extract can be used for rapid determination.
Apparatus
Vacuum pump assembly, buchner funnels, spatula, porcelein dish, filter
paper, pH meter, EC meter etc.,
Method
1. Weigh about 200 to 400 g of soil into a porcelein dish. The weight of the
soil used will depend on the volume of extract required. In general,
approximately one third of the water added is recovered in the saturation
extract.
2. Add sufficient deionized or distilled water while mixing to saturate the
soil sample. At saturation, the soil paste becomes glisten as it reflects
light, flows slightly when the dish is tapped and slides cleanly from the
spatula. A trench caved in the soil surface will readily close upon jarring
the container.
3. Allow the sample to stand for at least 2 to 4 hours and check to ensure that
saturation criteria are still met. If free water has accumulated on the
surface, add a weighted amount of soil and remix. If the soil has stiffened
or does not glisten, add distilled water and mix thoroughly.
4. After allowing to paste to stand for at least 2 to 4 hours, take the pH by pH
meter then transfer to a Buechner funnel fitted with highly retentive filter
paper. Apply vacuum and collect the extract in test tube until air passes
through the filter. Turbid filtrates should be discarded or refiltered. Add 1
drop of 0.1 % (NaPO3)6 solution per 25 mL of extract to prevent
precipitation of CaCO3.
5. Store extract at 4 0C until analyzed.
Results- (i) pHs =
8
(ii) ECe (dSm-1
)
Exercise [3] Determination of Ca2+ + Mg2+ in saturation extract of soil by
Versenate (EDTA) method
Principle
Ca + Mg in solution can be titrated with 0.01 N EDTA using Eriochrome
black T dye as indicator at pH 10 in the presence of ammonium chloride and
ammonium hydroxide buffer. At the end point colour changes from wine red to
blue or green. If calcium is present in the solution, this titration will estimate both
calcium and magnesium. Beyond pH 10 magnesium is not bound strongly to
Eriochrome black T indicator to give a distinct end point.
Reagents
1. EDTA or Versenate solution (0.01 N) : EDTA solution (0.01 N) : Take
2.0 g of versenate, dissolve in distilled water and make volume to 1 Litre.
Titrate it with 0.01 N calcium solution by the procedure discussed below
and make necessary dilution so that its normality is exactly equal to 0.01
N.
2. Ammonium chloride-ammonium hydroxide buffer: Dissolve 67.5 g of
ammonium chloride in 570 mL of concentrated ammonia and make to 1
litre volume.
3. Eriochrome black T indicator : Take 100 mL of ethanol and dissolve 4.5 g
of hydroxyl amine hydrochloride and 0.5 g of the eriochrome black T.
indicator. Hydroxylamine hydrochloride removes the interference of
manganese by keeping it in lower velency state (Mn++
). Or mix
thoroughly 0.5 gram of the indicator with 50 g of ammonium chloride.
4. Sodium cyanide solution (2%) or sodium diethyl dithiocarbamate crystals:
This is used to remove the interference of copper, cobalt and nickel.
9
Method
1. Pipette out 10 mL of aliquot (soil extract or irrigation water) in porcelein
dish containing not more than 0.1 meL-1
of Ca plus Mg. If the solution has
a higher concentration, it should be diluted.
2. Add 5 mL of ammonium chloride-ammonium hydroxide buffer. Now add
3-4 drops of Erichrome black T indicator.
3. Titrate this solution with 0.01 N versenate till the colour changes to bright
blue or green and no tinge of wine red colour remains behind.
Observations
S.No. Volume of
aliquot taken
(mL)
Burette reading Volume of
EDTA used
(mL)
Initial Final
1
2
3
Calculations
If N1 and V1 are normality (concentration of Ca2+
+ Mg2+
) and volume of
aliquot taken and N2V2 are the normality and volume of EDTA used,
respectively, then, N1V1 = N2V2
N2V2 Normality of EDTA x Vol. of EDTA
Or N1 = ------------------ = -------------------------------------------------------
V1 mL of aliquot taken
Here N1 = Normality = gram equivalents of Ca2+
plus Mg2 +
present in one Litre
of aliquot.
Hence,
10
Normality of EDTA x Vol. of EDTA
Ca2+
+ Mg2+
(meL-1
) = ------------------------------------------------x1000 = --------
Volume of aliquot taken
Ca2+
+ Mg2+
(ppm) = Ca2+
+ Mg2+
(meL-1
) x equivalent weight (32) = ------------
Results : (i) Concentration of Ca+ Mg (meL-1
) = --------------------------------
in soil extract
or concentration of Ca + Mg (ppm) = ------------------------------
in soil extract
11
Exercise [4]: Determination of Ca2+ in saturation extract of soil by Versenate
(EDTA) Method
This method, developed by Schwarzentach and Biederman, is very useful
on account of its accuracy, simplicity and speed.
Principle
The method is based on the fact that calcium, magnesium and a number of
other ions form stable complexes with versenate (ethylene diamine tetra-acetic
acid disodium salt) at different pH. Some elements like Sn, Cu, Zn, Fe, Mn may interfere in the determination of calcium and magnesium. Their interference is
prevented by the use of 2% NaCN solution or carbamate. Usually in irrigation
waters and water extracts of soil, the quantities of interfering ions are negligible
and can be neglected.
A known volume of the solution is titrated with standard versenate 0.01 N
solution using murexide (ammonium purpurate) indicator in the presence of
NaOH solution. The end point is a change of colour from orange red to purple at
pH 12 when the whole of calcium forms a complex with EDTA.
HOOCCH2 CH2COOH NaOOC-CH CH2COONa
N-CH2-CH2-N N-CH2-CH2-N
HOOCCH2 CH2COOH CH2 CH2
O = C-O O-C=O
EDTA EDTA Complex with metal ion
Reagents
1. Standard 0.01 N calcium solution : Take accurately 0.50 g of pure calcium
carbonate and dissolve it in 10 mL of 3 N HCl. Boil to expel CO2 and then
make the volume 1 Litre with distilled water.
2. EDTA solution (0.01 N) : Take 2.0 g of versenate, dissolve in distilled
water and make volume to 1 Littre. Titrate it with 0.01 N calcium solution
by the procedure discussed below and make necessary dilution so that its
normality is exactly equal to 0.01 N.
(Metal ion)
A
A
12
3. Murexide indicator powder : Take 0.2 g of murexide also known as
ammonium purpurate and mix it with 40 g of powdered potassium
sulphate. This indicator is not stored in the form of solution as it gets
oxidized.
4. Sodium diethyl dithiocarbamate crystals: Used to remove the interference
of other metal ions.
5. Sodium hydroxide 4 N : Prepare 16% NaOH solution by dissolving 160 g
of pure sodium hydroxide in water and make volume to 1 litre. This will
give pH 12.
Method
1. Take a suitable aliquot (5 or 10 mL) of the given solution of the soil
extract in porcelein dish and add 2-3 crystals of carbamate and 5 mL of
16% NaOH solution.
2. Add 40-50 mg of the indicator powder. Titrate it with 0.01 N EDTA
solution till the colour gradually changes from orange red to reddish violet
(purple). It is advised to add a drop of EDTA at every 5 or 10 seconds, as
the change of colour is not instantaneous.
3. The end point must be compared with a blank reading. If the solution is
over titrated, it should be back titrated with standard calcium solution and
record exact volume used.
Observations
S.No. Volume of
aliquot Taken
(mL)
Burette reading Volume of
EDTA used
(mL)
Initial Final
1
2
3
13
Calculations
Normality of EDTA x Vol. of EDTA
or Ca(meL-1
) = ------------------------------------------------------- x 1000
volume of aliquot taken
Ca++
(ppm) = meL-1
of Ca x equivalent weight of Ca2+
(20) = ------------------
Result :-
(i) Ca2+
concentration in saturation extract is------------------ meL-1
or-----------
ppm.
(ii) Concentration of Mg (meL-1
) = Ca + Mg (meL-1
) – Ca (meL-1
) =------------
or Concentration of Mg (ppm) = Ca + Mg (ppm) – Ca (ppm)=---------------
14
Exercise [5] : Determination of Na+ in saturation extract of soil by flame photometer
Principle:
Sodium is determined by flame photometer. Analysis through flame
photometer is based on the measurement of the intensity of characteristics line
emission given by the element to be determined. When a solution of salt is
sprayed into a flame, the salt gets separated into its component atoms because of
the high temperature. The energy provided by flame excites the atoms to higher
energy levels (the electrons of atom go to high energy level). When the electrons
return back to the ground or unexecited state, they emit radiation of characteristic
wave length (line emission spectrum). The intensity of these radiations is
proportional to the concentration of particular element in solution which is
measured through a photo cell in the flame photometer.
Equipment and Reagents
1. Flame photometer with Na filter
2. Volumetric flask (100 mL)
3. Sodium chloride standard solution :Dissolve 5.845 g of A.R. grade NaCl
in distilled water and make volume to one Litre. It will give 100 meL-1
of
sodium. This solution is treated as stock solution.
4. From this solution take 0, 1, 2.5, 5.0, 7.5 and 10 mL in volumetric flasks
of 100 mL capacity and make the volume by further adding distilled
water. This will give a series of standard solutions having 1, 2.5, 5.0, 7.5
and 10.0 meL-1
Na.
Method
1. Read the operation manual of flame photometer. Set the Na filter. Start
the compressor and light the burner of flame photometer. Keep air
pressure at 5 lbs and adjust the gas feeder so as to have a blue sharp
flame cones.
15
2. Adjust the zero reading of the meter by feeding distilled water. Now feed
standard sodium solution of the highest value in the standard series (10
meL-1
Na) and adjust the flame photometer to read full value of emission
in the scale i.e. 100 reading.
3. Feed different standard sodium solutions one by one and record the
emission value (reading) for each.
4. If concentration of Na is high in extract than dilute it by taking 10 mL
extract (aliquot) of sample in a 100 mL volumetric flask and make
volume 100 mL by distilled water.
5. Feed the diluted extract in flame photometer and note the reading.
Note: If flame photometer does not show reading of unknown, it indicates that
the concentration of Na in unknown (diluted extract or aliquot) solution is higher
and is out of the range of flame photometer scale. In such situation, further dilute
the extract (unknown solution) and take the reading.
Observations
Reading of known solutions (Standard solutions)
S.No. Concentration of Na in known
solution (meL-1
)
Reading on flame photometer
1 1.0
2 2.5
3 5.0
4 7.5
5 10.0
Reading of unknown solution = x
Plot a standard curve between concentration and readings of standard
sodium solutions. Obtain concentration of Na in unknown solution from the
standard curve.
16
Calculations
Na (meL-1
) in soil extract = Na (meL-1
) as obtained from curve x Dilution
factor, if any
100 (Here, volume of extract = 100; Aliquot taken = 10, hence, dilution factor is -------= 10)
10
Na (ppm) = meL-1
x equivalent weight of Na (23) =
Result : The concentration of Na+ in saturation extract is -------------------
meL-1
or -----------ppm.
17
Exercise [6] : Determination of K+ in saturation extract of soil by flame photometer
Principle
Potassium emits an yellow colour (404 millimicrons) flame when excited
in the flame. The intensity of emission is proportional to the concentration of
potassium in the sample.
Materials required
(i) Flame photometer (ii) Volumetric flasks 50, 100 and 1000 ml
(iii) 100 ml beaker and (iv) A.R. grade potassium chloride salt
Reagents
A. Ammonium acetate, approximately 1 N, To 700 or 800 ml. of water add 57
ml of concentrated acetic acid and then 68 ml of concentrated ammonium
hydroxide. Dilute to a volume of 1 litre and adjust to pH 7.0 by the addi tion
of more ammonium hydroxide or acetic acid.
B. Potassium chloride, 0.02 N. Dissolve 1.491 gm of dry potassium chloride in
water and dilute to a volume of exactly 1 litre.
C. Potassium chloride 0.02 N in 1 N ammonium acetate. Dissolve 1.491 gm of
dry potassium chloride in reagent A. Dilute to a volume of exactly 1 liter
with additional A.
D. Lithium chloride (0.05 N. Dissolve 2.12 gm of dry lithium chloride in water
and dilute to 1 liter.
Procedure
Using reagents B and D, prepare a series of standard KCl solutions, each
containing the same concentration of lithium chloride. Prepare a similar series of
standard potassium solutions using reagents C and D, and use A for dilution. The
concentrations of potassium chloride are 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5 and 2.0
meq L-1
. Calibrate the flame photometer for operation over the concentration
range 0 to 0.5 meq L-1
of potassium. Pipette an aliquot of the solution to be
18
analyse containing less than 0.1 meq L-1
of potassium into a 50 ml volumetric
flask. Add an amount of reagent D which when diluted to a volume of 50 ml, will
give a concentration of lithium chloride exactly equal to that in the standard
potassium chloride solutions. Dilute to volume with water or with A, if
ammonium acetate extracts are being analysed, mix and determine the potassium
concentration by use of the flame photometer and the appropriate calibration
curve.
Calculations
(me L-1
of K from calibration curve x 50)
K in soil extract (me L-1
) = ----------------------------------------------------
ml. in aliquot
Results:
K in soil extract (me L-1
) = ------------------------------
19
Exercise [7] : Determination of CO32- and HCO3
- in saturation extract of soil
Principle
Carbonate and bicarbonate in a solution can be determined by titrating the
solution against standard acid using phenolphthalein and methyl red respectively
as indicators. When the colour of phenolphthalein is disappeared, it indicates half
the neutralization of carbonate. At this stage methyl red i ndicator is added and
the titration continued. When the colour changes from yellow to rose red, it is the
end point for the complete neutralization of bicarbonate. The following equations
illustrate these changes:
2Na2CO3 + H2SO4 = 2Na2HCO3 + Na2SO4
(phenolphthalein = pink) (phenolphthalein = colourless)
2Na2HCO3 + H2SO4 = Na2SO4 + 2H2O + 2CO2
(methyl red = yellow) (methyl red = rose red)
Reagents
1. Standard H2SO4 (0.05 N) : Take 1.4 mL of concentrated H2SO4 (36 N)
with automatic pipette and dilute to one liter with distilled water.
2. Methyl red indicator (0.5%) : Dissolve 0.5 g dry methyl orange powder in
100 mL of 95% ethanol.
3. Phenolphthalein indicator (0.25%) : Dissolve 0.25 g of pure
phenolphthalein powder in 100 mL of 60% ethanol.
Method
1. Piptte out 10 mL of soil extract in a conical flask or in porcelein dish and
add 2-5 drops of phenolphthalein. Appearance of pink colour indicates the
presence of carbonates. Initial reading designate as I.
2. If carbonate is present, add 0.05 N H2SO4 from burette till the solution
becomes colourless. Record this reading (designate as II).
3. Add a few drops of methyl red indicator and titrate till the colour changes
from yellow to rose red.
20
4. Record this reading also (designate as III). Repeat this process a number
of times so as to get three concordant reading.
Observations Volume
of aliquot
taken
(mL)
Burette
reading
Vol. (mL) o f
H2SO4 used
for half
neutralization
of CO3=
(II – I) = x
Vol. (mL) o f
H2SO4 used for
complete
neutralization of
CO3 =2x
Vol. (mL) o f
H2SO4 used for
complete
neutralization of
CO3 plus HCO3
(III – I) = Y
Vol. (mL) o f
H2SO4 used for
neutralization of
HCO3=
Y - 2x
I II III
1
2
3
Calculations
(a) CO32-
in meL-1
If N1 and V1 are normality (concentration of CO32-
) and volume of aliquot
taken and N2V2 are the normality and volume of H2SO4 used respectively, then,
N2V2
N1V1 = N2V2 or N1 = -----------
V1
Here N1 = normality = gram equivalents of CO3-- present in one Litre of aliquot.
Hence, meL-1
of CO32-
is :
Normality of H2SO4 X Vol. of H2SO4 used 2x X 0.05 X 1000 CO3 (meL
-1) = ------------------------------------------------------- x 1000 = ----------------------------
Volume of aliquot taken Volume of aliquot taken
(b) HCO3- in meL
-1
Normality of H2SO4 X Volume of H2SO4 (Y-2x) X 0.05 X 1000
HCO3 (meL-1
) = ---------------------------------------------------- x 1000 = ------------------------------ Volume of aliquot taken Volume of aliquot taken
Result : Concentration of CO3 = ------------------------------meL-1
Concentration of HCO3- = -------------------------------meL
-1
21
Exercise [8] : Determination of Cl- in saturation extract of soil
Principle
Chloride in the extract of soil can be determined by titrating the extract
against standard AgNO3 solution using potassium chromate as indicator. There is
a formation of sparingly soluble brick red silver chromate precipitate at the end
point. Initially the Cl- ions are precipitated as AgCl. The dark brick red
precipitate as Ag2CrO4 formed just after the precipitation of AgCl is over. The
reactions involved are as under.
NaCl + AgNO3 = AgCl + NaNO3
(white ppt.)
K2CrO4 + 2AgNO3 = Ag2CrO4 + 2KNO3
(brick red ppt)
Reagents
1. 0.01 N sodium chloride : 0.585 g of NaCl (AR grade, derived at 80 0C for
1 hour) is dissolved in distilled water and made to one litre volume.
2. 0.01 N silver nitrate: Dissolve 1.6989 g of AgNO3 in distilled water and
make the volume upto 1 litre Standardize it against standard NaCl solution
(0.01 N) and keep in amber coloured bottle away from light.
3. Potassium chromate (K2CrO4) indicator solution (5%) : Dissolve 5 g of
K2CrO4 in about 75 mL distilled water and add saturated solution of
AgNO3 drop wise until a slight permanent red precipitate is formed. Filter
and dilute to 100 mL.
Method
1. In a clean titration flask or in proceeding dish take 10 mL of the soil
extract by pipette.
2. Add few (1-2) drops of potassium chromate indicator in extract taken.
3. Titrate with 0.01 N AgNO3 solution till a permanent brick red precipitate
persists. Take three concordant readings.
22
Observations
S.No. Volume of
aliquot taken
(mL)
Burette reading Volume of
AgNO3 used
(mL)
Initial Final
1
2
3
Calculations
Normality of AgNO3 x Vol. of AgNO3 x 1000
Cl (meL-1
) = ---------------------------------------------------------------- = -------------- volume of aliquot taken
Cl (ppm) = meL-1
of Cl x eq. Wt. of Cl (35.5) = -------------------------------
Result : Chloride (Cl-) concentration in saturation extract of soil is -----------
meL-1
or --------------------------ppm.
23
Exercise [9] : Determination of gypsum requirement of sodic soil
(Schoonover 1952)
The reclamation of alkali soil (pH in water suspension exceeding 8.5)
requires gypsum treatment for replacement of sodium ions from the exchange
complex. The sodium so released has to be leached out by flooding. The gypsum
requirement can be determined by adding a known excess of saturated solution of
gypsum (CaSO4.2H2O) to soil and estimating its unreacted amount by EDTA
(versenate) titration.
Reagents
(i) Saturated calcium sulphate solution : About 5g of pure CaSO4.2H2O
taken in one litre of distilled water, shake mechanically for 10 minutes
and filtered it through ordinary filter paper.
(ii) Ammonium chloride-ammonium hydroxide buffer of pH 10 : 67.5 g
pure ammonium chloride dissolved in 570 mL of concentrated
ammonia solution (sp. Gr. 0.88) and diluted to one litre with distilled
water and adjusted to pH 10 (Caution : Liquid ammonia should be
refrigerated before opening the bottle).
(iii) Eriochrome black T indicator : 0.5 g of eriochrome black T and 4.5 g
of hydroxylamine hydrochloride (AR)dissolved in 100 mL of 95%
ethyl alcohol.
(iv) 0.01 N calcium chloride solution : 0.500 g of AR grade calcium
carbonate taken in little excess of AR HCl (about 10 mL of dil. acid)
and the solution made upto one litre with distilled water.
(v) Standard versenate (EDTA) solution 0.01 N : 2.0 g of ethylene-
diamine-tetra acetic acid disodium salt and 0.05 g of MgCl2 (AR)
dissolved in water and diluted to 1 litre; the solution to be standardized
against 0.01 N calcium chloride.
24
Procedure
5g of soil is taken in a 250 mL conical flask to which 100 mL of the
saturated CaSO4 solution is poured in, shaken for 5 minutes and filtered through
Whatman No. 1. After rejecting first few mL, 5mL of the extract is pipetted into
a 100 mL flask or porcelein dish and diluted to about 25 mL with distilled water.
One mL of NH4Cl – NH4OH buffer and 3 to 4 drops of eriochrome black T
indicator are added and titrated with the standard EDTA solution untill the colour
changes from wine red to blue. Similarly, 5 mL of the saturated CaSO4 solution
is titrated separately to determine the Ca concentration as described in exercise 3.
Observations
(i) Ca + Mg in soil extract
S.No. Vol. of aliquot
taken (mL)
Burette reading Vol. of EDTA
used (mL) Initial Final
1
2
3
(ii) Ca in saturated gypsum solution
S.No. Vol. of
saturated
gypsum
solution taken
(mL)
Burette reading Vol. of EDTA
used (mL) Initial Final
1
2
3
25
Calculation
normality of the EDTA X mL of the EDTA X 1000
Ca (meL-1
) = ---------------------------------------------------------------------------
mL of saturated CaSO4 Solution taken
= --------------------------------------
normality of the EDTA X mL of the EDTA X 1000 Ca + Mg (meL
-1) = -------------------------------------------------------------------------------------------
mL of soil extract (filtrate) taken
= -----------------------------------------------
Gypsum requirement in meL-1
100 g = [(Ca concentration in saturated gypsum
solution (meL-1
) minus Ca – Mg concentration in filtrate (meL-1
)] x 2
= -----------------------------------------
Gypsum requirement : in metric tons (tonnes) per hectare (15 cm soil depth)
= 1.72 x G.R. (meL-1
100 g of soil)
= -----------------------------------
Result : Gypsum requirement of soil is ------------------------t ha-1
.
26
Exercise [10] : Determination of lime requirement of acid soils
While a slightly acidic condition is often favourable for crop growth but a
higher degree of soil acidity mostly exerts an adverse effect on crop growth and
hence such acid soils need lime application to improve the productivity. The
quantity of lime required to bring the pH to the desired level will vary according
to the nature of the soil. Number of laboratory methods are available for this
purpose. The procedure given by Shoemaker et al. (1961) is being widely
followed for determining the lime requirement of acid soils of pH less than 6.0.
A glass electrode pH meter is the instrument needed.
Reagent
Extractant buffer: 1.8g nitrophenol, 2.5 mL triethanolanine, 3.0 g
potassium chromate, 2.0g calcium acetate and 53.1 g calcium chloride dihydrate
(all chemically pure) are dissolved in a litre of water and the pH adjusted to 7.5
with dilute NaOH solution.
Procedure
To 5g of air-dry soil taken in a dry 50 mL beaker, 5 mL distilled water
and 10 mL of the extractant buffer are added and stirred continuously for 10
minutes or intermittently for 20 minutes. The pH of the suspension is determined
on the basis of which the requirement of lime is read from the following table.
The values are given in tons of pure calcium carbonate per acre required to bring
the soil to the pH indicated and are to be converted to their equivalents of the
form of agricultural lime to be used. For expressing in metric units i.e.,
tonnes/ha, the figures are to be multiplied by 2.43.
27
Lime requirement to bring the soil to the desired pH level according to pH
value of the soil buffer suspension
pH of soil Buffer
Suspension
Lime required to bring the soil to indicated pH
(in tons/acre of pure calcium carbonate)
pH 6.0 pH 6.4 pH 6.8
6.7 1.0 1.2 1.4
6.6 1.4 1.7 1.9
6.5 1.8 2.2 2.5
6.4 2.3 2.7 3.1
6.3 2.7 3.2 3.7
6.2 3.1 3.7 4.2
6.1 3.5 4.2 4.8
6.0 3.9 4.7 5.4
5.9 4.4 5.2 6.0
5.8 4.8 5.7 6.5
5.7 5.2 6.2 7.1
5.6 5.6 6.7 7.7
5.5 6.0 7.2 8.3
5.4 6.5 7.7 8.9
5.3 6.9 8.2 9.4
5.2 7.4 8.6 10.0
5.1 7.8 9.1 10.6
5.0 8.2 9.6 11.2
4.9 8.6 10.1 11.8
4.8 9.1 10.6 12.4
Result : The lime requirement of acid soil is ----------------------------tons/acre.
The lime requirement of acid soil (t ha-1
) = tons/acre x 2.43 = ---------------
28
Exercise [11]: Determination of saturated hydraulic
conductivity of an undisturbed soil
Principle: The saturated hydraulic conductivity of soil refers to the readiness
with which it transmit water. Mathematically it may be expressed
as
QL
K = ----------- HAT
Where,
K = Saturated hydraulic conductivity (cm/h)
L = Length of soil column (cm)
A = Cross sectional area ( r2-cm)
H = Pressure head (cm)
T = Time in minutes
Q = Quantity of water conducted (m2)
Saturated hydraulic conductivity (K) is the proportionality constant in
Darcy’s law indicating the ability of soil to transmit flowing liquid.
Material required
(i) Core sampler
(ii) Hammer or pressing unit
(iii) Inner brass sectional cylinder
(iv) Sharp and rigid knife or spatula
(v) Permeability apparatus with complete accessories
Procedure
1. Press the core sampler in to the soil to the desired depth and is
carefully removed to preserved a known volume of sample as it existed
in situ.
29
2. The samples holding main ring alongwith guard ring (brass sectional
core) is removed from the cone cutter by inverting the core. The brass
sectional core is slipped off and extraneous soil is trimmed with a
knife.
3. A piece of filter is held to the bottom of the core followed by brass
sieve.
4. Place the core in the tough with sufficient water to reach just below the
brim of core.
5. Saturate the soil sample completely (atleast 10 hours).
6. Transfer the saturated soil alongwith sieve on funnel of permeability
apparatus rack. (Fig.-1)
Fig. 1 : Constant head permeability apparatus (undisturbed)
7. Place aluminum ring of 2 cm height on top of each core making a
water proof attachment of rubber tube.
8. Adjust the siphons to deliver water to each core (6 core at a time may
accommodate) and maintain constant head of water on all cores by
same level.
9. Collect the water from each cores by placing the measuring beakers
below funnels.
10. Measure the quantity of percolating water in three 30 minutes.
30
11. Remove the aluminum ring and transfer the soil in moisture boxes for
the determination of bulk density.
Observation
Quantity of water percolated
S.No. I
30 minutes
II
30 minutes
III
30 minutes
Mean
Normal
Problematic
Managed
Calculation
QL Saturated hydraulic conductivity (cm h
-1) =------
HAT
Result :
(i) Saturated hydraulic conductivity of normal soil ----------- (cm h-1
)
(ii) Saturated hydraulic conductivity of problematic soil --------- cm h-1
(iii) Saturated hydraulic conductivity of managed soil --------- cm h-1
31
Exercise [12]: Determination of bulk density of an
undisturbed soil by core sampler method
Principle :
Bulk density is the ratio of the mass to the bulk volume in a soil sample
which is expressed in terms of Mgm-3
. The mass is determined after drying the
soil sample to a constant weight in a over at 1050 to 110
0C for 24 hours.
Materials Required
1. Core sampler.
2. Hammer or pressing unit.
3. Brass sectional cylinder.
4. Sharp and rigid knife or spatula.
5. Moisture box.
6. Oven.
7. Physical/ electric balance, sensitive to 0.01 gram.
Procedure :
1. A double- Cylinder, hammer- driven core sampler is pressed into the
soil to the desired depth and is carefully removed to preserve a known
volume of sample as it existed in situ.
2. The sample holding main ring along with guard rings are removed
from the container by inverting the cores. The container is slipped off
and soil is trimmed with a knife, or
3. After determination of saturated hydraulic conductivity, the entire
mass of the soil in the main ring is drawn in the moisture box and kept
for oven drying at 1050C for 24 hours.
32
Observations and calculations :
Soil sample Weight of
empty
moisture
box (MB)
(g)
A
Weight of
oven dried
soil + MB
(g)
B
Volume of
core- r2h
(cm3)
C
Weight of
oven dried
soil
B-A
Bulk
density
(Mg m-3
)
B-A
C
Normal
Problematic
Managed
Results :
(i) The bulk density of an undisturbed normal soil ……… Mg m-3
(ii) The bulk density of an undisturbed problematic soil …….. Mg m-3
(iii) The bulk density of an undisturbed managed soil ……… mg m-3
33
Exercise [13]: Determination of infiltration rate of soil by double ring
infiltrometer
Principle
The infiltration rate of a soil is a measure of its capacity to take in or
absorb water applied to the soil surface. Initially, the rate at which water enters
the soil is very high. This rate decreases with time until a relatively constant
value is attained. This is generally referred to as the basic infiltration rate or the
basic intake rate of soil.
Infiltration rate, ‘t’ has units of volume per unit area per unit time or is
simply expressed as depth per unit time. Cumulative infiltration, I, is defined as
the total accumulated infiltrated depth of water within a specific time.
Cumulative infiltration is the integration of infiltration rate and conversely,
infiltration is the derivative of cumulative infiltration, that is,
t
I = i dt …(7) 0
and
dl
i(t) = -------------- …(8)
dt
The infiltration rate of soil cannot be directly related to its hydraulic
conductivity due to the changing hydraulic gradients and soil water content
during the process. But this rate does vary with the saturated hydraulic
conductivity of the soil layers. Layers with low hydraulic conductivity values,
located either at or below the surface will limit the infiltration rate and in
particular, the basic infiltration rate. Surface crusts, silt deposits, machinery
tracks, plow layers or clay pans will also influence the infiltration process. The
initial soil water content will always affect the infiltration rate, drier soils
34
exhibiting higher initial infiltration rates compared to moist soils. A saturated soil
will infiltrate water at about the basic infiltration rate.
Ring infiltrometer
In case a metallic ring (or cylinder) is being used for infiltration
measurements, after water has penetrated into the soil to a depth below the
bottom of the ring, it will start spreading laterally as well as vertically. This will
also affect the infiltration rates. To minimize this effect, a buffer pond can be
created by constructing an earthen dike around the ring or by driving a larger
diameter metal ring concentric with the ring infiltrometer. The use of two
concentric metal rings to determine the infiltration characteristics is, therefore
preferred and referred to as the double ring infiltrometer method. This method
measures the vertical rate of entry of water into the soil surface.
Equipment and materials required
Double ring infiltrometer (30 and 45 cm in diameters and 30 cm height)
with hammer, Hook’s gauge, time, source of water, plastic sheet.
Procedure
Infiltration rate is generally determined in the field using the cylinder or
ring infiltrometer as it is popularity called.
Step I: The spot at which the infiltration rate is to be determined is
carefully cleaned of weeds and leveled.
Step II: The two metal concentric rings (or cylinders) are gradually
hammered into the soil. Care is taken to ensure that the rings are
pushed vertically downwards with least possible disturbance to the
soil surface. The rings should be pushed to a depth a atleast 15 cm
into the soil.
Step III: Water is first poured into the outer (buffer) ring. When water is
being applied initially into the inner ring, a piece of plastic or
35
polythene is placed inside the ring to prevent any disturbance at or
crusting of the soil surface. This sheet is subsequently removed and
the initial reading of the water level is recorded immediately.
Step IV: Water is maintained in the buffer ring at about the same depth as
inside the ring. The level of water in the rings is maintained
between 6-8 cm or the depth of water generally existing during
application of irrigation water.
Step V: Observations of the level of water in the ring are taken periodically.
Initially the intervals between two consecutive observations are
kept short. They become larger with passage of time. They are
recorded as indicated in the observation sheet.
Step IV: Step IV and V are repeated until two consecutive readings of
infiltration rates are obtained.
Step:V Plot a graph of CI and IR on a log-log paper
Observation and calculation for managed soil :
Time in
minutes
Infiltration readings (cm) Cumulative
infiltration
(cm)
Infiltration
rate (cm h-1
)=
Difference in
CI x 60/ time
interval (min)
Initial Final Difference
0-5
5-10
10-20
20-30
30-60
60-90
90-120
120-150
150-180
36
Observation and calculation for problematic soil :
Time in
minutes
Infiltration readings (cm) Cumulative
infiltration
(cm)
Infiltration
rate (cm h-1
)=
Difference in
CI x 60/ time
interval (min)
Initial Final Difference
0-5
5-10
10-20
20-30
30-60
60-90
90-120
120-150
150-180
Results
Soil site Infiltration rate (cm h-1
) Cumulative infiltration (cm)
Normal
Problematic
Managed
37
Exercise [14]: Determination of maximum water holding capacity of the
soil
Principle
It is some times called the maximum water retentive capacity. It is defined
as the amount of moisture in a soil when its total pore space is completely filled
with water. This happens when a this layer of soil is allowed to absorb water
from a free water surface. Saturation percentage or maximum retentive capacity
are the equivalent term for maximum water holding capacity.
Maximum amount of water observed by soil
MWHC(%) = ------------------------------------------------------------------- x 100
Oven dry weight of soil
Saturation percentage is approximately 4 times of wilting point and 2
times of the field capacity.
Apparatus
(i) Keen’s box- It is a cylindrical brass dish having internal diameter of 5.6
cm and height of 1.6 cm. The bottom is perforated with holes of 0.75 mm
in diameter & 4 m apart.
(ii) Filter paper circles corresponding to internal diameter of disc.
(iii) Petri dish having diameter greater than that of Keen’s box
Procedure
(i) Grind the dry soil to pass through 2.0 mm sieve.
(ii) Weight empty keen box with the filter paper fitted in the bottom.
(iii) Fill it with soil while tapping until it is full of soil to ensure a level surface.
(iv) Place the box in a petri dish having water upto the depth of ½ cm.
Maintain the water depth till the whole mass in moistened.
(v) Leave the box in petri dish, over night (24 hours).
38
(vi) Next day remove the box from the petri dish, carefully with the box and
allow to drain for 30 minutes.
(vii) Take the weight of soil with keen box.
(viii) Place the box in an oven at 105 0C and weight till constant weight.
(ix) Weight 5 dry filter paper, saturate them with water and weight again. Find
out the average amount of water absorbed by one filter paper circle.
Observation and calculation
Soil sample Weight of empty keen box with filter (g) (A)
Weight of box + saturated soil + filter paper (g) (B)
Weight of box + oven dried soil + filter paper (g) (C)
Weight of water absorbed by one filter circle (g) (D)
B-C-D MWHC(%) = ------- x 100
C-A
Normal
Problematic
Managed
Results :
(i) The MWHC of normal soil is ----------------------------%
(ii) The MWHC of problematic soil is ----------------------%
(iii) The MWHC of managed soil is --------------------------%
39
Exercise [15]: Determination of field capacity (FC) of soil in field
condition
Principle
Field Capacity (FC) is the term used to describe the maximum amount of
water that an initially saturated soil will retain after the gravitational water has
drained out. It does not generally correspond to a fixed soil water suction (or
potential) which varies from 1/10 bar (10 kPa) for coarse textured soils to 1/3 bar
(33 kPa) for fine textured soils.
Equipment and materials
Balance, oven plastic sheet, straw mulch, source of water, soil sampling
auger, aluminium (moisture) box and oven.
Procedure
Step : A representative bare plot, eg. 3 m x 3 m, is leveled properly free
from and bunded (30 cm high) from all sides.
Step II: Water is continuously pounded on the bare plot for several days till
the profile is fully wetted upto at least 30 cm below the proposed
sampling depth.
Step III: Immediately after ponding is over, the plot is covered with a
polythene sheet to prevent evaporation until the observations are
over. Mulch is also applied over the sheet to avoid any excessive
heating of the soil surface.
Step IV: Soil samples are taken at various depths after 24 or 48 hours until
the moisture content at successive samplings agree to equal within
1 per cent (Data is recorded on the observation sheet provided).
40
Observation and calculation
Soil sample Estimation of moisture Soil water
content
(g/g)
B-C
------
C-A
Soil water
content
(%)
B-C
------x100
C-A
Bulk
density
Mg m-3
Soil water
contents
B-C
------xBd
C-A
(cm3/cm
3)
MB +
moist soil
(g)
B
M.B +
dry soil
(g)
C
Weight of
empty MB
(g)
A
Normal
Problematic
Managed
Result :-
(i) Field capacity of normal soil …………………………….
(ii) Field capacity of problematic soil …………………………….
(iii) Field capacity of managed soil …………………………….
41
Exercise [16]: Determination of available water storage capacity of soil
by pressure plate (membrane) apparatus (Richard, L.A.
Weaver, L.R. 1943 and Richards, 1947).
Filed capacity is defined as the amount of water held in the soil after the
excess gravitational water has drained away under free drainage and the rate of
downward movement of water has materially ceased. FC is the upper limit of
available soil water for plant use. The \ force by which water is retained in the
soil at FC is 1/3 bar. The wilting point refers to that soil moisture content at
which soil is unable to supply water at a rate of sufficient to maintain turgor in
plants after being saturated with water and the plants wilt permanently. Wilting
point is a lower limit of available water.
Equipments and materials:
1. Pressure plate- (membrane apparatus complete with all the
fittings with 1/3 and 15 bar ceramic plate cells.
2. ‘O’ rings or Rubber soil retaining rings of 1 cm height and 6 cm
in diameter that hold about 25 gm of soil.
3. Balance 4. Moisture box 5. Drying oven
6. Sieve (2 mm size) 7. Soil sampling auger 8. Pipette
Procedure
1. Prepare duplicate 25 gm samples that have been passed through a 2
mm round – hole sieve for each soil type.
2. Place soil samples in rubber rings on the ceramic plate in the ring.
Each ceramic plate can accommodate in soil samples.
3. Put the ceramic plate containing soil samples in an enamel tray.
4. Fill the water in the enamel tray.
42
5. Allow the samples to stand over night with an excess of water on the
plate.
6. When the samples are ready for the extractor, remove the excess water
from the ceramic plate with a pipette.
7. Mount the ceramic plate in the extractor and connect up with out flow
tubes.
8. Put a triangular support in the bottom of the vessel (extractor).
9. Mount lid and screw down clamping bolts.
10. Build up the pressure in the extractor to the equilibrium value slowly.
For 1/3 bar, 5 PSI pressure should be maintain in the extractor and for
15 bar, the pressure in the extractor to be is set at 220-250 PSI (lbs).
11. At any given pressure in the chamber, soil moisture will flow from soil
particles and out through the ceramic plates until an equilibrium is
reached and the flow of moisture is ceases. This can be judged by
connecting each out flow tube to the tip of a burette by rub tube for
collecting the water.
12. The burette can be read periodically and if burette reading observed
constant over a period of many hours, the equilibrium is attained
between air pressure and soil suction.
13. After the equilibrium has been attained, the samples are removed,
samples are immediately transferred to moisture boxes, dried in oven
at 105 0C and moisture content is determined.
43
Observations
Observations Tension (Bar)
1/3 (FC) 15 bar (PWP)
Normal Problematic Managed Normal Problematic Managed
(A) Weight of
empty moisture
box
(B) Weight of
M.B. + moisture
soil
(C) Weight of
MB + even dried
soil
Calculation
1. Weight of moisture in soil = B-C (g) ---------------------
2. Weight of dry soil = C-A (g) --------------------
B-C
3. Moisture content (%) = --------- x 100 -------------------
C-A
4. Available water in soil = Moisture content (%) at 1/3 bar -
Moisture content (%) at 15 bar =--------
-------------------------------
FC –PWP x BD x soil depth (cm)
5. Available water storage capacity= ----------------------------------------- = ---
(AWSC) of soil (cm per depth) 100
44
Result
Normal soil Problematic soil Managed soil
1/3 bar 15 bar 1/3 bar 15 bar 1/3 bar 15 bar
1. Moisture content (%)
2. Available water (%)
3. BD (Mg m-3
)
4. AWSC (cm/depth)
45
Exercise [17]: Measurement of oxygen diffusion rate by platinum
microelectrode method (Lemon and Erickson, 1952)
Principle :
The principle of this method is that when a certain electrical potential is
applied between a reference electrode and a platinum micro-electrode inserted in
the soil (Fig. - 2), oxygen is reduced at the surface of platinum micro-electrode
causing an electric current flow between the two electrodes. The reduction of
oxygen platinum micro-electrode causing an electric current flow between the
two electrodes. The reduction of oxygen at the surface of platinum electrode
depends on the amount of oxygen diffusing to that electrode through the water
layer surrounding the electrode. The current flowing between the electrodes is
proportional to the rate of oxygen reduction that is the current is governed by the
rate of oxygen diffusing to the electrode. Thus by this technique it is possible to
estimate the rate of oxygen diffusion as it would reach a plant root.
Fig. 2 : Schematic diagram of oxygen diffusion meter
46
The general reaction taking place at the platinum micro-electrode surface
in the reduction of oxygen occurs in two steps, involving two electrons in each
step. The reaction in two different media are given below :
In an acid medium :
O2+2H++2e
- =H2O2
H2O2 + 2H+ +2e
- = 2H2O
O2+4H+ + 4e
-- 2H2O
In neutral or alkaline medium :
O2+2H2O+2e- =H2O2 + 2OH
-
H2O2 + 2e- = 2OH
-
O2+2H2O + 4e- = 4OH-
Thus each molecule of oxygen which diffuses to the surface of the micro-
electrode takes up four electrons and reacts with hydrogen ions to form water in
an acid solution or reacts with water to form hydroxyl ion in an alkaline solution.
The electric current flowing between the two electrodes is related to the
flux of oxygen fx and the relation is stated below;
it
it x 10-6
= nFAfx or fx = --------
nFA
where, it is current in micro amperes at time ‘t’ in seconds, n is number of
electrons required to reduce one molecule of oxygen (n=4); F is Faraday constant
(96500 coulombs); fx is oxygen flux at the electrode surface per second at time
‘t’ in moles/cm2 sec and A is area of the electrode surface in [2h + r) ] and the
oxygen diffusion rate (ODR) is calculated by the following relation.
47
it x 10-6
ODR = ------------------------ moles/cm2/sec
4 x A 96500 x A
it x 10-6
x 60 x 32
ODR = ----------------------------- g/cm2/min
4 x A x 95600 x A
it x 10-6
x 60 x 32 x 106
or ODR = ----------------------------- g/cm2/min
4 x 96500 x A
The factor 60 x 32 x 106 is due to conversion of the results into minutes
and micrograms.
Material and equipments :
1. Platinum microelectrode assembly
2. Core sampler
3. KCL wetted blotting paper
4. Porous cup
5. Cable and alligator clips
Procedure :
1. Collect undisturbed core samples and saturated them (follow the
procedure of bulk density and saturated hydraulic conductivity).
2. Equilibrate soil cores at 60 cm soil moisture tension by putting them
on tension table.
3. Place the cores in porous cup having KCl wetted blotting paper.
4. Turn the knob to calibrate and adjust the potentiometer at 6.5
microamperes (0.65v).
5. place 5 electrode in each core about 4 cm deep and make all the
connection with the help of cable and alligators
6. Turn the knob of ‘ON’ and start timing
48
7. After 5 minutes, turn the selector switch to one and read the diffusion
rate of electrode one.
8. Repeat this procedure for each electrodes. Measurement of ODR can
similarly be made in situ also, in which case electrodes should be
inserted 6 to 10 cm deep in soil.
Observations and calculations
Soil samples Current (microampare) in the reduction of
O2 at platinum electrode
ODR
it
fx = ----------
nFA
(g/cm2/min)
Normal
Problematic
Managed
Results :
(i) The oxygen diffusion rate of normal soil is ……………… g/cm2/min
(ii) The oxygen diffusion rate of problematic soil is ………….. g/cm2/min
(iii) The oxygen diffusion rate of managed soil is …………….. g/cm2/min
Problems :
1. A platinum electrode wire having a diameter of 0.5 mm and length
(outside the electrode) of 5mm recorded the current throughout the
circuit of 5 micrometer. Calculate the ODR and state the aeration
status of the soil and suitability of crops in this field.
Solution ; We know that
49
it x 10-6
it x 10-6
x 60 x 32
ODR = mole/cm2/sec = g/cm
2/min
4 x A x 95600 A x 4 x 95600
A = r (2h + r)
= 0.025 (2 x 0.5 + 0.025) 22/7 [ h = 5 mm = 0.5 cm and r =
0.5/2mm = 0.025cm]
= 0.025 x 1.025 x 22/7 cm2 = 0.08 cm
2
5x 10-6
x 60 x 32
ODR = g/cm2/min
4 x 0.08 x 95600
= 31.38 x 10-8
g/cm2/min
So, the aeration status of the soil is very good. Crops favouring well
aerated condition can be grown to this field.
2. In the above ODR meter if the current throughout the circuit is 0.4
micrometer what would be the aeration status of the soil and what crop should be
recommended for the field.
Solution :
0.4 x 10-6
x 60 x 32 786 x 10-6
ODR = g/cm2/min = g/cm
2/min
4 x 0.08 x 95600 305920
= 0.251 x 10-8
g/cm2/min
So, the aeration status of the soil is very poor and crops favouring poorly
aerated condition like paddy may be recommended for this field. Aeration status
of these soils must be improved by soil management processes for growing well
aerated crops like maize, wheat etc.
50
Exercise [18]: Estimation of water stable soil aggregates
Principle
The wet sieving method involves equilibrating a given amount of soil
aggregates in a nest of stand sieving in water, for a given length of time,
followed by the collection of aggregates plus the coarse materials, retained on
each sieve and their weight. Finally, the soil mass, retained on each sieve, is
dispersed in H2O2 and HCl, and passed through individual sieves to account for
the coarse soil fractions, which, otherwise, might be included wrongly, while
reporting the mean weight diameter, and the percent of the total aggregates, in
different size fractions, of the soil mass.
Apparatus and Equipment
Standard sieves-2 sets (5.0, 2.0, 1.0, 0.5, 0.2 and 0.1 mm); Yoder
apparatus, physical balance, oven; desiccators; watch glasses (8 cm); wash-
bottle, can boxes.
Regents
Hydrogen peroxide; HCl (0.1 N)
Procedure
Take about, 300 gm of air-dry solid clods. Break them into smaller
aggregates by pulling them apart with hand, such that they pass through
8.0 mm screen, and are retained on 5.0 mm screen. Do not break them too
small. Large gravel or roots should be removed.
Weight 50 gm aggregates (5.0-8.0 mm) in three watch glasses. Keep one
of them in oven at 105 0C for water content determination, and use the
other two for analysis in duplicate.
Arrange two sets of six sieves (5.0, 2.0, 1.0, 0.5, 0.2 and 0.1 mm) in such a
way that the uppermost sieve has the largest mesh size, and the sieve, at
the bottom, should have the smallest mesh size.
51
A. Aggregate sample : Spread sample aggregate evenly on the top sieve and
spray 5-10 ml of salt-free water on them. Wait for 3-5 min., spray another 5-10
ml of water again, and wait for further 3-5 min.
Transfer the nest of sieves to the drum of the sieve, shake and clamp them
in position securely. Fill the drum with salt-free water upto a level slightly
below the top screen, when the sieves are in the highest position (turn the
pully of the shaker slowly with hand to attain the highest position)
Lower the sieves to the lowest position, and wet the aggregates for 10 min.
Full more water in the drum so that the aggregates are just covered with
water when sieves are in the highest position.
Switch on the oscillator and let the sieves oscillate in water for 30 min.,
with a frequency of 30-35 cycles/ min, through a stroke length of about 3.8
cm, and check that the aggregates on the top sieve remain immersed
throughout the full stroke.
Take out the nest of sieves, let the water drain for a few min in an inclined
position, remove excess water from the bottom of screens with absorbent
tissue and place them on paper sheets. Let the aggregates on each sieve dry
and harden in air.
Dry the soil in an oven at a temperature not higher than 75 0C because
high temperatures cause some soils to adhere to the sieves. Drying of the
aggregate surfaces takes between 20 and 40 min, depending on the soil.
When dry transfer the soil from each sieve separately to can boxes, dry
overnight at 105 0C in an oven and weigh.
B. Dispersed sample : In order to determine how much of the soil, retained
on the sieves, represents aggregates and how much is gravel or sand, transfer the
aggregates of each to 250 ml breakers separately, and disperse them with H2O2
and HCl treatments. Pass the dispersed aggregates again through the same sieves
on which they were retained earlier. Collect the unaggregated primary particles,
52
from each sieve, in can boxes, as per the procedure outlined in the preceding
paragraph, and record their oven-dry weight.
Calculate the percentage of aggregated soil particles on different sieves.
Plot a graph between the accumulated percentage of the soil, remaining on
each sieve on ordinate, and the upper limit of each size fraction on
abscissa.
Measure the area under the curve, which is a representative of the mean
weight diameter (M.W.D. or weighted diameter) of aggregates.
Find out the mean weight diameter of aggregates in mm by computation
also and report the results as M.W.D. and percent aggregation.
Observations
a. sample No. I II
b. Air dry weight of sample = 50 gm 50 gm
c. Water content in solids = - - %
d. Frequency of oscillation = - - min.
e. Stroke length = - - cm
f. Oven-dry weight of the aggregated and the unaggregated
particles=
- - gm
St.
No.
Particle size
range (mm)
Particle
diameter
(>mm)
Wt. of particles retained on sieves (gm)
Before dispersion After dispersion
Sample-I Sample-II Sample-I Sample-II
A B C D E
1. > 5.0 5.0
2. 5.0-2.0 2.0
3. 1.0-0.5 1.0
4. 0.5-0.25 0.5
5. 0.25-0.10 0.10
53
Calculations
Sample No. I II
g. Oven-dry weight of sample [(100/(100+c)] x 50] = ------------- ----------gm
h. Per cent Aggregation
Particle
diameter (>mm)
Wt. of aggregated
particles
Percent of total soil
sample
Accumulated
percentage
Sample-I Sample-II Sample-I Sample-II Sample-I Sample-II
C F = (D-E) G = (F x 100)/g II
n
i. Mean weight diameter (MWD) : X1W1 = ----------mm i=1
where, n = 6 (number of size fractions, i.e. 5.0 to 0.1 mm)
X = the mean diameter of fraction ‘t’
W = the proportion by weight of a given size fraction of aggregates :F/g
MWD of sample-I : = -------------- mm
MWD of sample-II: = --------------- mm
MWD from graph : Sample-I; Sample II = ---------------- mm
Results
1. Percent aggregation (mean) greater than 0.1 mm = ---------------%
2. MWD (mean of two observations from calculations) = ---------------mm
3. MWD (mean of two observations from graphs) = ---------------mm